The transportation of chemicals in the liquid or gas state in pipes exhibits numerous advantages: it is more economical, once the infrastructure has been installed, it makes possible the transportation of large volumes, it ensures great security of supply, by virtue of a steady output, and, finally, it is a safer means than rail or road.
The use is known, for the transportation of liquid or gas products, of metal or plastic pipes or also of metal pipes coated with one or more polymeric layers. Depending on the fluid to be transported, these pipes have to meet multiple requirements, in particular as regards the properties of mechanical strength (in particular impact strength), of elasticity, of creep strength, of fatigue strength, of resistance to swelling, of chemical resistance (resistance to corrosion, to oxidation, to ozone, to chlorinated products and the like) and of thermal resistance.
For example, pipes comprising one or more metal components guaranteeing the mechanical stiffness but which are not leaktight to the fluids transported (for example, components made of steel or of cast iron), and also various layers based on polymeric compositions, for ensuring leaktightness to the fluids transported and also thermal insulation, are known. Typically, in the case of the polymeric layers, the thickness/diameter ratio is of the order of 1/10. These polymeric compositions can be based on polyethylene but this limits the operating temperature of the pipes to at most 60° C. They can also be based on fluorinated polymers, such as polyvinylidene fluoride (PVDF), suitable for higher operating temperatures, up to 130° C., and exhibiting good chemical resistance and good thermal resistance. However, PVDF is very stiff and, for this reason, PVDF homopolymers are often formulated or used as a blend with copolymers based on vinylidene fluoride (VDF) and optionally plasticizer, in order to reduce the stiffness thereof
Flexible pipes are also used for the transportation of oil or natural gas extracted from offshore or onshore deposits. These pipes are formed of multilayer structures comprising in particular polymeric sheaths and metal reinforcing layers or reinforcing layers made of composite materials.
The flexible pipes comprise, from the inside toward the outside:                at least one internal leaktightness pipe in contact with the transported fluid, consisting of a polymeric material,        one or more reinforcing layers surrounding said internal leaktightness pipe, and        an external protected sheath.        
The document BE 832851 describes fluorinated elastomers comprising a molar proportion of 50 to 85% of VDF and of 15 to 25% of hexafluoropropylene (HFP), i.e. a proportion by weight of 47 to 71% of VDF and of 29 to 53% of HFP, which are used for the manufacture of molded PVDF bodies comprising from 1 to 30% by weight of fluorinated elastomer. However, such compositions are of limited extrudability and do not make possible the manufacture of pipes having a thickness/diameter ratio of close to 1/10.
The document EP 1 342 752 describes PVDF-based compositions comprising: (A) a PVDF homopolymer or a VDF-based copolymer; (B) a fluorinated elastomer; (C) optionally a plasticizer. The fluorinated elastomer (B) is present at a content of 0.5 to 10 parts by weight per 100 parts of homopolymer or copolymer (A) and of 0 to 10 parts by weight of a plasticizer (C) with the additional condition that the sum of (B) plus (C) is from 0.5 to 10.5 parts by weight. These compositions correspond to the following proportions by weight: 89.5 to 90.5% of a PVDF homopolymer or a VDF-based copolymer (A); 0.5 to 9% of a fluorinated elastomer (B); 0 to 9% of a plasticizer (C). The contents of fluorinated elastomer, of less than 10%, do not make it possible to confer, on the finished product, characteristics of sufficient fatigue strength for the applications described above.
The document EP 608 639 describes polymeric compositions comprising, by weight, from 60 to 80% of PVDF, from 20 to 40% of a thermoplastic copolymer of VDF and of another fluorinated comonomer (present at a content of 5 to 25% in the copolymer) and from 5 to 20% of a plasticizer (with respect to the sum of the PVDF and copolymer). VDF/HFP copolymers appear among the thermoplastic copolymers envisaged. The HFP contents shown in the copolymers which are disclosed in the examples are of the order of 10%. However, these compositions undergo an extraction of the plasticizer on contact with some chemical substances. Consequently, the sheaths produced with these compositions do not retain a stable geometry, their volume and thus their thickness having a tendency to decrease with time.
This phenomenon can have a detrimental effect on the leaktightness of the end fittings located at the two ends of the pipe. This is because the leaktightness between, on the one hand, the end of the pressure sheath and, on the other hand, the body of the end fitting is produced by a tightening process, that is to say a process of tightening the sheath between two concentric parts supported respectively by the internal and external faces of said sheath.
An excessive decrease in the thickness of the sheath can thus have the effect of degrading the tightening pressure and the leaktightness of the end fitting.
The document WO 2006/045753 describes polymeric compositions comprising a PVDF homopolymer, a thermoplastic fluorinated copolymer and a third component which can be a plasticizer at a content of 5% by weight at most, a perfluorinated polymer or a VDF polymer of low molecular weight. The thermoplastic fluorinated copolymer can, for example, be a copolymer of VDF and of another fluorinated comonomer, which can be present in a content of 5 to 25%. Characteristically, the blend of PVDF homopolymer and of fluorinated copolymer has a mean intrinsic viscosity of less than 2 dl/g. Furthermore, the polymeric composition exhibits an apparent melt viscosity of less than or equal to 60 000 Pa·s, at a rate gradient of 1 s−1. However, the fluorinated polymeric compounds of the tubular structures produced with such compositions are not entirely satisfactory. In particular, when the structures are subjected to repeated movements due to marine currents, the fatigue strength of these fluorinated polymeric compounds is judged insufficient.
The document FR 2987624 describes compositions comprising a PVDF homopolymer, an elastomeric fluorinated copolymer and a plasticizer. The copolymer exhibits elastomeric properties which it confers on the fluorinated polymeric compound and which makes it possible not only to increase the fatigue strength of said compound but also to improve the resistance to cold. This is an essential difference from the subject matter of the document WO 2006/045753, where compositions capable of forming nonelastomeric fluorinated polymeric compounds are specifically desired. However, the extraction of the plasticizer which takes place during the lifetime of the tubular structure, in addition to modifying the geometry of the sheaths (variation in volume and thickness), as already mentioned above, gradually results in a loss in the properties initially introduced by the plasticization (flexibility, toughness), consequently limiting the lifetime of the articles based on these compositions.
The object of the present invention is thus to develop a novel polymeric composition based on a specific PVDF homopolymer, on an elastomeric fluorinated copolymer and on a third fluorinated component of low viscosity, said composition exhibiting mechanical and chemical properties which are improved with respect to the known compositions and which do not change during the life of the pipe in service, in order to manufacture pipes which remain mechanically reliable over the long term, in particular for the pressure single-sheath application used in the offshore field.